The invention relates to a process for the reparation of aluminium-strontium master alloys, to master alloys thus obtained and to the use of these master alloys as structure refiner during the solidification of molten aluminium-silicon alloys.
Aluminium-silicon alloys are widely used for the production of cast products as aircraft parts, internal combustion engine parts as pistons and valve sleeves etc. To obtain cast products of a suitable (high) quality it is essential to add a structure refiner to the molten alloy to induce the formation of relatively small silicon crystals during the solidification. The thus obtained cast products show increased mechanical properties, ductility and strength when compared with the case that a structure refiner is not used.
In this specification the term structure refiner is used for a compound or composition which, after addition and mixing and/or dissolution in a molten metal or alloy, either as such or as a newly formed compound, induces during solidification the formation of smaller crystals than would have been the case when the structure refiner would not have been used.
Heretofore, sodium has been used as a structure refiner for the aforesaid aluminium-silicon alloys, especially eutectic or hypo-eutectic aluminium-silicon alloys, i.e. alloys containing up to about 12% by weight of silicium. More recently strontium has been used instead of sodium because it gives a better structure refining effect than sodium, together with a more economical (limited burnoff loss compared with sodium) and less dangerous process.
During the solidification of hypo-eutectic aluminium-silicon alloys first primary aluminium crystals are formed until the eutectic composition is obtained, whereafter simultaneously aluminium crystals together with silicon crystals are formed. The silicon crystals show an acicular form and are fairly large when no structure refiner is used. When a structure refiner is used these silicon crystals are relatively small and show a fibrous character, resulting in the above described improved properties.
It is presumed that upon dissolving an aluminium-strontium master alloy small particles of aluminium-strontium intermetallics (Al.sub.4 Sr) are liberated which at their turn dissolve and thus provide strontium in solution, whereafter the strontium during the solidification increases the number of silicon crystals substantially, resulting in a large number of small crystals instead of a small number of large crystals.
Strontium may be added to the aluminium-silicon melt as a pure metal or as a master alloy. As the addition of metallic strontium is quite troublesome, the strontium is predominantly added in the form of master alloys. In this respect reference is made to U.S. Pat. No. 4,009,026, describing a strontium-silicon-aluminium master alloy, and U.S. Pat. No. 3,567,429, describing a strontium-silicon master alloy. The processes for the preparation of the master alloys described in the above mentioned patents, however, are quite laborious and expensive. Further, the thus obtained master alloys have contact times of between five and thirty minutes before the refining effect is fully obtained. These alloys have a microstructure in which especially the AlSr.sub.4 particles are coarse. This results in the long contact times and is furthermore detrimental to the ductility of the product. Attempts have therefore been made to prepare quick dissolving aluminium-strontium master alloys to allow in-line (addition in the launder) feeding and which have sufficient ductility to enable coiling and decoiling.
The dissolution velocity of conventionally cast aluminium-strontium master alloys, however, is low, especially when the amount of strontium in the alloy is more than 5% by weight. Furthermore, these alloys are usually very brittle, which makes it impossible to use conventional coil feeders. See for instance U.S. Pat. No. 4,576,791. Especially the low dissolving velocity is a clear disadvantage as the master alloys are preferably added just immediately before casting in view of the high oxidation velocity of strontium. This helds especially in the case of launder feeders.